1. Field of the Invention
The present invention relates to a satellite broadcast receiver, and, more specifically, to a satellite broadcast receiver which is capable of receiving not only video signals but also audio SCPC (Single Channel Per Carrier) signals.
An SCPC system is known as a system in which one carrier is used per one signal channel and a plurality of those carriers are sent simultaneously, thereby transmitting a plurality of channels.
2. Description of the Prior Art
Recently, broadcast services have been expanded such that television signals and audio signals can be broadcast via satellite. Following this trend, various proposals have been made regarding receivers for television and audio signals broadcast from satellites.
Japanese Patent Public Disclosure No. 4-33948 published on Nov. 26, 1992 and Japanese Patent Public Disclosure No. 5-49032 published on Feb. 26, 1993 discloses that some circuit components are commonly used both for a satellite broadcast receiver and an audio PCM receiver.
An audio SCPC is one of the services provided by an SCPC signal utilizing a satellite. Usually, a receiver dedicated for the reception of this audio SCPC signal is required. Such a SCPC-dedicated receiver cannot receive satellite broadcast signal because the audio SCPC service employs a signal format quite different from that of the satellite broadcast.
FIG. 1 schematically illustrates available frequency bands for satellite broadcast and the audio SCPC service. FIG. 1(a) illustrates frequency bands of vertical polarization and horizontal polarization signals in satellite broadcast channels separated from each other by 40 MHz. FIG. 1(b) illustrates a frequency allocation of an audio SCPC signal. As can be seen from this figure, a plurality of FM carriers are distributed within a frequency band of 40 MHz. Adjacent FM carriers are respectively spaced, for example, by 260 kHz and a frequency band of each FM carrier is 60 kHz in the case of an audio SCPC signal.
Since both satellite broadcast and audio SCPC services are provided through satellite broadcast, there is a user's need to develop a product which enables both satellite broadcast and the audio SCPC signals to be received by the same receiver. However, accommodating a satellite broadcast receiving circuit and an SCPC receiving circuit within a single receiver set results in a complicated receiver structure with high receiver costs.
Japanese Patent Public Disclosure No. 5-64101 published on Mar. 12, 1993 proposes a receiver in which a tuner device is commonly used for a demodulator of frequency-modulated video signals (video and audio) and a demodulator of MSK-modulated musical signals both sent via a satellite.
Incorporation of an audio SCPC receiving circuit into an ordinary satellite broadcast receiving system causes the following problems.
An output frequency of an LNB (low Noise Block Down Converter) provided in an antenna device of a satellite broadcast receiving system changes by approximately 3 MHz in one day under the influence of temperature. This frequency drift is larger than the receiving frequency bandwidth of an audio SCPC signal. However, even if an AFC circuit is used to conduct automatic frequency control so as to receive a desired SCPC signal, automatic frequency control over a wide frequency band cannot be achieved using a conventional AFC circuit due to limitations imposed by a pull-in range and a reception frequency band.